The chemical state of a gas phase sample is formed by vaporized or gaseous chemical species mixed with an ambient medium, typically environmental air. Instead of air, the medium can be process gases or vacuum. The detector is used to detect and identify defined chemical species in the defined surrounding media.
Characteristic for a chemical detector is its capability to convert a chemical state to an electrical signal and transmit the signal for further processing. Typically it is aimed at performing both qualitative and quantitative determination of defined chemical species in a defined ambient medium. In that case, a technical concern is that the detector output is not completely specific, but possesses sensitivity to other chemical species than those aimed at. This behaviour is often referred as cross-sensitivity and typically leads to false positive identification.
Two fundamental ways to reduce the cross-sensitivity problem of the chemical detectors are (i) the development of more specific sensors (where the sensor is considered as the first part of a measuring chain converting the input variable into a signal suitable for measurement) or (ii) performing chemical separation before detection. Typical solutions for the latter case are using chromatography techniques or filtration or controlled adsorption-desorption techniques or applying sample preparation procedures including for example dissolution, phase separation, extraction, chemical derivatization and ion exchange. In the case of detecting the gas phase chemical state, and more preferably when detecting minor constituents in the environmental air by a portable detector, the sample preparation steps are less favoured as they are difficult to automatize, difficult to mobilize and also time consuming, and thus not suitable for fast real-time monitoring.
Of the remaining possibilities, chromatography is a well-known method in analytical chemistry for performing chemical separation. Gas chromatography (GC) is a method of choice for the separation of stable and volatile compounds as well as of gas phase samples. The method accomplishes chemical separation by partitioning the components of a mixture between a mobile gas phase and a stationary solid or liquid phase held on a solid support. In a fixed chromatographic system the retention time (which is the time passing when the sample travels from the inlet through the column to the detector) is constant for a particular analyte and, therefore, can be used to identify it. Thus, although chromatography is primarily a separation technique, it is possible to identify the separated compounds of a complex sample by their retention times. The process is carried out in a GC instrument consisting typically of a sample feed arrangement, a carrier gas and its flow controller unit(s), one or more columns inside a chamber (typically equipped with a thermostat), and one or more of said chemical detectors.
A crucial technical component of GC in respect to separation power and thus resolution of the analysis is the column. Two basic columns can be distinguished: (i) the packed column and (ii) the open tubular or so called capillary column. The packed columns are constructed from tubing of e.g. stainless steel, nickel or glass, inner diameters ranging typically from 1 mm to 10 mm. The columns are packed with an inert support powder, usually diatomaceous earth with an average internal pore diameter of 1-10 μm and a particle size of 100-200 μm. The second column type, the open tubular capillary column, has a narrow internal diameter of 10-1000 μm. It is typically constructed of fused silica (a very high purity glass) while the outer wall is protected by hard and tough polymer, like polyimide. Furthermore, they are characteristically of tubular shape with an unrestricted flow path in the middle of the column. The inner fused silica surface is chemically modified by various type of coatings or films which provide so called stationary phases with different polarity and thus selectivity for the separation process. The stationary phase can be a liquid layer or a thin film typically made of polymer such as polysiloxane, silicone or polyamide, optionally functionalised in different ways. Factors such as chemistry, microstructure, morphology and thickness of the stationary phase film influence the total separation power of the column.
Of the column types, the open tubular capillary column is favoured in analytical chemistry due to its better separation power per total analysis time, better long-term stability and higher quality due to a more reproducible manufacturing process.
The use of open tubular GC capillary tubes in combination with various portable chemical detectors is well-known in the art as can be concluded from the following citations: U.S. Pat. No. 5,114,439 and U.S. Pat. No. 5,856,616 disclose the use of compact sized and low power consuming GC columns for portable applications. Also WO9941601 discloses the use of a combined specific sampling system and a low power consuming GC column. Furthermore, U.S. Pat. No. 4,888,295 discloses the use of “a commercially available” GC column in combination with detector formed by an array of electrochemical sensors (CPS), and U.S. Pat. No. 6,354,160 discloses the use of a GC column in parallel with SAW-sensor based detectors, where the open tubular GC columns may also be those formed on silicon wafers.
Applying the GC method in portable devices, and preferably in hand-held size devices, requires devices which are low-power consuming, light and compact sized and have a fast detection while still maintaining a high resolution through high separation power. So far, the improvements of portable devices have mainly concerned the use of high column temperatures as well as improvements in temperature control and in the construction of the heating system. Furthermore, prior art improvements have concerned modifications of the carrier gas flow as well as design of special sampling and detecting systems.
Other ways for improving the GC method's suitability to portable applications have included shorter columns and columns with smaller inner diameter in order to enhance the efficiency and the speed of the analyses. However, these improvements will lead to reduced separation or alternatively, they will reduce the sample volume and increase significantly the power requirement and thus the cost and dimensions of the pump due to increased pressure drop in the column. The drawbacks of using a low sample volume is that it typically leads to weakened response by the detector and increased sensitivity to local variations in the sample leading thus poorer accuracy in retention time. Also controlling small volumes of fluid can be a technically demanding as well as an expensive solution.
These drawbacks have been overcome by using a column which comprises a bundle of open tubular capillaries. See e.g. Baumbach et al. (1997) and Baumbach et al. (2000).
Such columns are manufactured and/or sold by only a few companies, namely, Alltech Associates Inc. (Deerfield, Ill., USA), ChemSpace s.r.o (Pardubice, Czech Republic), Sibertech (Novosibirsk, Russia). The advantages of multicapillary columns are that they provide short retention times and thus fast detection times at sufficiently high resolution and separation capability. Furthermore, they retain high efficiency over a wide range of carrier gas flow rates and, thus, compared to conventional single capillary columns, they can be operated with larger sampling volumes that are easy to inject and detect.
Thus, the properties of the claimed multicapillary column makes it ideal for a hand-portable gas chromatograph.
However, since multicapillary columns are typically formed by hundreds of single capillary columns, it is difficult to obtain uniform thermal distribution with low power consumption for the sufficiently massive bundles, which reduces the accuracy of the GC analysis.
Even though multicapillary GC columns facilitate much higher sampling flow rate (or carrier gas flow rate) through the column than a single open tubular GC column, the compatible gas flow rate for conventional multicapillary columns still remains below 300 ml/min. In some detector types this flow rate can be still far too low. Such detector is, for example, a hyphenated multisensor-ion mobility spectrometer designed for detecting gaseous chemical species in the environmental air by direct flow-through principle as described in references WO9416320 and Utriainen et al. (2003).
The detector employs a special type of ion mobility spectrometer (IMS) referred to as aspiration condenser type or open loop type IMS combined with other sensors such as semiconductor gas sensors, temperature and humidity sensors. The detector is manufactured for hand-held and portable chemical detector devices under trademarks such as ChemPro100, M90-D1-C (Environics Oy, Mikkeli, Finland) and MultiIMS (Dräger Safety, Lubeck, Germany). Further characteristic for this detector is that it employs continuous, typically 800-3500 ml/min, preferably 1000-2000 ml/min flow-through providing thus good statistical sampling accuracy and fast response and recovery times which are all essential features especially when aiming at to provide reliable early warning of the presence of toxic substances in the air. Characteristic feature for this detector is also that the sensitivity depends on flow rate in such manner that the higher flow rate is favored. Other characteristic features of the detector are the sensitivity to rapid flow (and pressure) changes and rapid and large humidity and temperature changes.